It happened in sheep and goats, and now it’s happening in cattle. Parasites are developing resistance to the drugs we use to fight them.

The problem in U.S. cattle herds has not, in most cases, become severe just yet, but scientists fear without preventive action, cattle parasites could become like those in small ruminants, where available drugs have almost no benefit in many herds.

The path to resistance among cattle worms resembles that of bacteria becoming resistant to antibiotics. With every treatment, a few target organisms survive due to a genetic tolerance to the drug. Those organisms pass on their tolerance to off-spring, and resistance gradually becomes the norm.

Use dewormers with proven efficacy, use the full recommended dose, avoid using pour-on formulations for internal parasites due to inconsistent dosing and efficacy, and rotate between classes of products, advises John Maas, DVM, who ranches in northern California and Oregon, and spent more than 20 years as an Extension veterinarian for the University of California, Davis. Pasture management and rotational grazing also help, he adds. Above all, work with a veterinarian to monitor the effects of your program and determine which products to use and the best treatment timing.

To develop a strategy for preventing the emergence of multidrug-resistant worms in your herd, University of Georgia veterinarian and parasite specialist Ray Kaplan emphasizes the following three components:

Refugia. This concept means leaving some animals untreated to maintain a population of susceptible worms. It also can involve leaving some pastures as refuges for susceptible worms rather than trying to keep every pasture clean.

Combination treatments. Using two or more classes of dewormers in a single treatment helps ensure a high parasite kill-rate, slowing the emergence of resistant worms.

Research on the use of refugia in cattle is sparse, says Louisiana State University veterinarian Christine Navarre, but in sheep and goats, where the resistance issue is much more widespread and severe, the concept has well-documented benefits.

Combination treatments also show promise, Navarre adds, but for now there are no combination products on the market in the U.S., so producers need to work with a veterinarian to select and administer the drugs. Higher treatment cost could deter producers, but she points out the combination treatments work best in a system that includes refugia, meaning fewer cattle treated.

At the least, producers can be more selective in the cattle they treat. Navarre suggests focusing on calves, heifers and young cows because older cows develop tolerance to most internal parasites. Treating fewer cattle, and lighter cattle, could help balance the cost of using two or more products.

Maas believes the improvement in efficacy can help slow the emergence of resistant pathogens, but says producers will be most likely to adopt the practice after they see signs of resistance, due to the increased cost. In some herds with resistance, Maas says, older products such as levamisole can provide good efficacy alone or in combination with more modern drugs.